Pages

Sunday, January 24, 2010

Reflections on the Sun

Last week I was in Great Britain (as you'll know if you follow me on Twitter). Even when you're flying long-distance Westbound a commercial plane doesn't catch up with the setting sun. Eg the flight from Frankfurt to Toronto takes 9 hours while you gain only 6 time-zones. But when flying South-West from Sweden on a winter afternoon one can amazingly enough see the sun rise again above the horizon. Winters here in Stockholm are cold and dark. Plenty of time to be reminded that life on earth would not be possible without a gigantic ball of hot plasma that our planet happens to be orbiting around.

We're so used to the sun that we often forget what a fascinating object it really is. Far from being the dull blob that it appears from far away, it's 1030 kg of nuclear matter with temperatures ranging from 5,000 K at the surface to 107 K at the core. Some months ago, during Nordita's program on "Solar and stellar dynamos and cycles" in a talk on Helioseismology, I saw this video showing a solar quake, waves on the sun's surface:

This quake from July 1996 was triggered by a solar flare in its center that was recorded just prior to the quake. Not the newest news, but I still think this is totally amazing. There's also a lot of physics in here. Unfortunately, I wasn't able to find the real-time scale is for the video, but I think it's roughly an hour. The actual size of the image shown is 100,000 km in each direction. The data was taken with the Michelson Doppler Imager of NASA's Solar and Heliospheric Observatory (SOHO) mission which basically measures the velocity perpendicular to the sun's surface by use of the Doppler shift in spectral lines. You can find a better resolution of the picture with a brief description of the event on this website.

It is interesting to note, and you can see this on the crappy video already, that unlike water waves you'd see in a puddle, the waves on the sun's surface increase their velocity with time (by roughly factor 10 for what is shown in the video). The explanation for this is that the waves are not surface waves, but pressure waves propagating into the sun's interior. Unlike the puddle the sun is a ball and its density increases towards the middle. With increasing density, the velocity of the waves (essentially the sound velocity) increases. The waves are reflected (similar to light-reflection/refraction on planar surfaces) and appear back on the sun's surface as outgoing rings whose outward velocity increases due to the geometry of the wavefronts and the density gradient. (For more details, the interested reader is referred to astro-ph/0601006 and references therein.)

So next time you look at the sun recall it's a giant ball of plasma held together by gravity, an every-day display of fascinating physics.

Quibble: "With increasing density, the velocity of the waves (essentially the sound velocity) increases." - one would think that with more mass/unit volume to move, the velocity of sound would go down with increasing density. Isn't the velocity increase with depth because of the increasing pressure?

A thought experiment that would possibly be useful is hitting a block of wood on a pendulum. When you hit one side, the other side "seems" to move instantaneously, but of course it doesn't, just very very quickly, too fast for us to notice in the macroscopic world. First the wood compresses oh so slightly, then the other side feels the vibration from the hit side, then moves.

I knew I would use my Mechanical Engineering on this site one day.

But, the Sun, as you say Bee, is endlessly fascinating. Most fascinating to me is how we humans figured out fusion and the exact process the Sun uses to fuse Hydrogen into Helium. It wasn't that long ago was it, in historical time? When was that, exactly, and who did that? It's elementary Quantum Mechanics Applications, but my mind seems to have been filled with "Falsification of Decoherence" issues lately ... for some reason. :-)

The three different kinds of waves that helioseismologists measure or look for are: acoustic, gravity, and surface gravity waves. These three waves generate p modes, g modes, and f modes, respectively, as resonant modes of oscillation because the Sun acts as a resonant cavity. There are about 10^7 p and f modes alone. [Harvey, 1995, pp. 33]. Each oscillation mode is sampling different parts of the solar interior. The spectrum of the detected oscillations arises from modes with periods ranging from about 1.5 minutes to about 20 minutes and with horizontal wavelengths of between less then a few thousand kilometers to the length of the solar globe [Gough and Toomre, p. 627, 1991].

You might recognize the owner of this site I keep a picture on my index because I am partial to the method to discerning how a solar flare can be forecast.

Hearing the Universe's shape or it's topology?:)It just another way in which can perceive the universe. In terms of Lagrangian, in concert, as well with three body problem, you get to envision the universe in a way that Wayne Hu spoke of.

If you sprinkle fine sand uniformly over a drumhead and then make it vibrate, the grains of sand will collect in characteristic spots and figures, called Chladni patterns. These patterns reveal much information about the size and the shape of the drum and the elasticity of its membrane. In particular, the distribution of spots depends not only on the way the drum vibrated initially but also on the global shape of the drum, because the waves will be reflected differently according to whether the edge of the drumhead is a circle, an ellipse, a square, or some other shape.

In cosmology, the early Universe was crossed by real acoustic waves generated soon after Big Bang. Such vibrations left their imprints 300 000 years later as tiny density fluctuations in the primordial plasma. Hot and cold spots in the present-day 2.7 K CMB radiation reveal those density fluctuations. Thus the CMB temperature fluctuations look like Chaldni patterns resulting from a complicated three-dimensional drumhead that....See:Vibrations of the Cosmic Drumhead

Sunspots are basically 2D bubbles of plasma formed under surface of Sun. When such bubble ruptures, it creates a protuberance splash and waves around it. In this context, nothing is quite interesting about it - but mass/energy scale of this event.

What is more interesting is the mechanism of sunspot formation and its eleven years period. In my opinion it's generated by Corriolis force, which is switching circulation beneath surface of Sun. The direction of this force is driven by location of center of mass of whole solar system, although the period of Jupiter planet is most important there.

Maybe ancient astrologues were quite right, while deriving the changes of climate and human psychics from from conjunctions of planets (wars for sources in particular).

/*..The Sol system is endlessly boring..*/Not quite, for example the axial tilt of planets could be driven by dark matter distribution in solar system. Such phenomena manifest itself just after long time, when various irregularities of planetary motion become blurred by gravitational waves and tidal forces. Young planetary systems are more dynamic, but they're still randomized by noise - so they're less complex, in fact.

That’s certainly a neat video of a sun quake, which is a phenomena I had never heard of before. Apparently as you point out such waves are very infrequent and solar flare activity is not enough on its own to explain their appearance. Howvere I must admit to not understanding their explanation for the increase of the waves velocity. The way I see it with the high gravitational force at the surface the translation of energy in terms of wave height would be strongly resisted with the tenancy then to increase the wave length which would increase the horizontal speed as result. Anyway with the explanations of the wave’s cause they offer I would have called it a Solar Tsunami rather thena sun quake.

The gravitational force on the surface of the sun is not much larger than on the Earth. It's a factor 30 or so. That's enough to be uncomfortable for humans, but it's only one order of magnitude difference (I suspect the temperature is much more uncomfortable ;-)). For this reason, the gravitational force doesn't play much of a role for these waves altogether. You might find it helpful to read the section on "Types of solar oscillations" in the Wikipedia-entry on Helioseismology. As to the increase in the velocity, it might be helpful if you draw a picture, it would look similar to this picture which shows p-waves in the Earth, except that the sun doesn't have a solid crust and the density changes more smoothly. The wave propagates towards the center and out again, but since their velocity in the denser region is higher, on the surface it looks like the wave is accelerating. Best,

Steven: Yes, that's true, nobody really "knows," but you'd really have to totally mess up the density of state for nuclear matter to get the sun have a solid core. The situation is more murky with the Earth I think. Yes, strange things could happen at high density and high temperature, but please keep in mind that compared to densities and temperatures created in heavy-ion collisions (disclaimer: so far as it's appropriate to talk about these quantities) 10^7 Kelvin isn't much. We have a pretty good understanding about processes in this energy regime. The really difficult part I'd think is the plasma dynamics. Best,

Helioseismic holography, introduced in detail by Lindsey and Braun for the purpose of far-side (magnetic) imaging,[7] a special case of phase-sensitive holography. The idea is to use the wavefield on the visible disk to learn about active regions on the far-side of the Sun.

Bee, I'm not really worried about the Sun's core. What little we know about the inside of the Sun, good ol' Sol, our lifegiver (except for certain parts of Greenland during the winter when the lack of same leads to depression, alcoholism, and suicide among the native Inuit) comes from extrapolating backwards of the considerable, and considerably increasing, knowledge database of what happens on the outside of the sun's atmosphere.

FAR more interesting to me is the whole neutrino bath we are awash in, and the detectors to learn more about them, thereby.

We know a LOT, yet ... are so ignorant, as every answer seems to spawn 4 new questions, etc. Heady and wonderful stuff, but a subject best left to experts.

I know about Plasma, good old ionized gas, but I'm more in tune with Heat Transfer (Conduction, Convection, Radiation), being a subfield of Thermodynamics, being a sub-field of Mechanical Engineering, as they relate to Quantum Mechanics, being a sub-field of Mechanics, also being a subfield of Mechanical Engineering.

We be "gearheads", sure, but some of those gears can get pretty small. :-P

But what I mostly know about MechE is that Entropy, our little-god-baby, and that which we worship, the 2nd law, throws more monkey wrenches into the works than theorists and geometers such as I also fancy myself, can dream of.

Well, I'm not the officially declared representative of Europe, but Madonna is one of the few entertainment "stars" that I have respect for. It's not so much that I like her music (the songs she did when I was a teenager I found quite silly), but she has constantly reinvented herself in contrast to many, many others who have one success and then repeat the same thing over and over again. Who is Paris Hilton? ;-)

Yes, this was what triggered this post. For what I heard though, there's been a lot of progress understanding the plasma dynamics, solar cycles, sunspots etc, and also a lot of data (check the SOHO website I linked to, amazing pictures there!). Still many open questions though (esp regarding the cycles). Best,

Well, I'm not the officially declared representative of Europe, but Madonna is one of the few entertainment "stars" that I have respect for. It's not so much that I like her music (the songs she did when I was a teenager I found quite silly), but she has constantly reinvented herself in contrast to many, many others who have one success and then repeat the same thing over and over again.

Agreed, she is very intelligent and a marketing genius. The ability to re-invent one's self in light of new data is important across many fields.

Who is Paris Hilton? ;-)

LOL! Although I despise the expression "Ignorance is bliss", in some cases is does apply, and Paris Hilton is the textbook application.

So go ahead and look her up on Google if you dare, but don't say I didn't warn you. Basically, she's a "celebutante," a famous person with no talent whatsoever who is famous for making herself famous. It gets worst from there. Spare yourself.

Steve: Yeah, it was a joke that would be impossible to understand for you, sorry. I so rarely read the entertainment section of the newspaper and never watch TV that a lot of stories blissfully pass me by. So there have been some incidents where I, totally unaware of what the world seems to have been talking about, had to ask who is Paris Hilton? And in some other case who is Martha Steward? (Meanwhile I know, thanks to Google.) Not particularly proud of it, but it's something my friends like to make jokes of. Best,

Thanks, Bee. Let us remind ourself that sarcasm rarely translates well to the written word. I spread that idea around quite a bit myself yet have to re-learn it, dam.

Solar cycles are fascinating, but neutrino oscillations are doubly so IMO. What Pauli started is for all intents and purposes still in its infancy, but crackerjack astronomers (not speculative Cosmologists like Carroll) are making great headway. I must remind myself to be patient, but it's hard sometimes.

Steven: Sure, neutrino oscillations are fascinating, but by themselves I think quite well understood. Questions remaining: what's the absolute masses, is there a right-handed neutrino, are there possibly additional sterile neutrinos? And can we finally get over LSND? Best,

I suspect it has something to do with Gravity and Energy. They seem to be interrelated. For some reason I can conceive of and possibly accept a Higgs Field but not a Higgs Boson. I'm probably wrong ... but it nags me. What do you think of Randall-Sundrum and Steinhardt-Turok Ekpyrotic?

is there a right-handed neutrino,

I don't know. Perhaps we're moving to the right so our weak-force neutrinos seem to move left? See Guth et. al. and to my way of thinking which I'm open to exploring, there may be a possible VERY large 5th dimension that is invisible to us because it is so large and affects every point in our peculiar local guthian neighborhood spacetime, such that we see the variable as a constant and nothing more.

are there possibly additional sterile neutrinos?

I've entertained the notion. Does that have to do with a fourth family? Or is it superstrings fantasy? I forget which, but I think its one of either.

In any event, I am a Quantum Gravitationist Geometer at heart, much like you and Lee, and I'm recently returned from a year-long vacation trying to learn the basics/cutting edge of Quantum Mechanics, so I do not know, but am willing to learn.

Of course, the greatest puzzles are Dark Matter and Dark Energy. Don't have the answer, but I'm betting Geometry does. Don't know, but keeping an open mind. Call me the "Anti-Lubos" in that regard.

They oscillate because the mass eigenstates are not the same as the flavor eigenstates. Nothing mysterious about that. It would be more mysterious if they were the same. The big question is of course why do the mixing angles have the values they have. But then the SM is full of why-questions of that sort. Fact is, the mixing angles and \Delta m^2's are meanwhile very well known. Given how small the mass differences are it's quite amazing actually how well they are known! (Incidentally, I wrote a blogpost Neutrinos for Beginners a long time ago, but I suspect you know everything I explained there already.) The masses I've been referring to are the eigenvalues of the mixing matrix. Best,

Steven: Forgot, see here for LSND. It's a neutrino experiment that (in)famously had results that didn't fit with all other experiments (and thus were generally completely omitted in the global fits). There's been a lot of controversy about that, and loads of papers discussing whether it's a bug or a feature. It's been tested with the MiniBooNE experiment recently, but for whatever reason MiniBooNE didn't exactly reproduce the LSND experimental setting, so some people still think there might be something odd in the data (more papers...) Anyway, so the situation still isn't entirely settled, but almost so I'd say. Best,

/*...You know why they oscillate between the families, do you..*/You should first have understanding, what the particle families are, but I doubt, some mainstream physical theory could give you an answer in intuitive way. Formal theories are answering formal questions, whereas nonformal ones can answer nonformal questions and their predictability scope doesn't overlap too much.

In aether theory all particles are product of nested condensation of aether and they undergo repeated evaporation and condensation like density fluctuations inside of gas. At the case of lightweight neutrino such breathing could be observed most easily. These undulations occur even for Falaco solitons (vortex rings) at the water surface. You can think about it like about undulation along time dimension(s), not just spatial ones.

In AWT the mass is the product of surface curvature of objects. Every blob of vacuum foam with more dense core has a positive mass, every "bubble" has a negative rest mass.

This doesn't mean, objects of finite size must have always nonzero rest mass, because space-time is not insintrically flat. For example the photons with shorter wavelength then those of cosmic microwave radiation have positive mass, the CMB photons have zero rest mass and photons of larger wavelength then some 2 cm are effectively a tachyons.

/*... the very point of this post was to explain that the waves you see in the video are not "splash" waves on the surface .... I don't see how what you say explains that the velocity of waves on the surface increases... */

Because this is what surface capillary waves usually do as the result of their dispersion - they're gradually changing from transversal waves into gravity waves (not to be confused with gravitational ones). It's true, dispersion is a bulk effect of extradimensions - but it doesn't mean necessarily, such wave is spreading through bulk phase in the way, you're describing.

The picture linked illustrates, wavelength of ripples increases with distance from source of wave, too.

This video footage illustrates clearly, what bulk waves really do, when they reach water surface from underwater: they create an analogy of cosmic microwave noise at the surface, because they're spreading faster, then the surface waves.

http://www.youtube.com/watch?v=SHoXJuv8tDM

In aether theory gravitational waves are such "underwater" waves and they manifest itself just by CMB noise inside of our spacetime. Mainstram physicists should take courses of classical mechanics obligatory before learning any math.

When the waves are longer, then the capillary waves, the same mechanism leads to gradual decrease of wavelength of waves (and the their speed as well). And all the above is valid just for incompressible fluids, for compressible ones everything the situation is even more complex.

So at the case of solar plasma explanation of Bee can still hold truth, because highly turbulent surface of Sun may not be able to spread some transversal waves at all. But it's not so easy to decide it just from the fact, speed of waves increases with distance, because capillary waves at water surface are of the same qualitative behavior.

Zephir: The neutrino can be its own antiparticle. I already explained to Phil above that yes, there are also gravity waves on the sun but their amplitude is much smaller than the observed one. I don't see how what you say explains the observation. If you have a better model for the solar dynamics than the presently accepted one, why don't you write a paper on it. Finally, please omit further comments with the only intent to advertise your theory of whatever. Thanks,

Today scientists and philosophers from many different fields are doing research about consciousness, searching for a fundamental scientific theory that explains this phenomenon in our universe. Do you think we need physics to explay one or all apects of consciousness phenomena?

To come back to the topic of the post − about sound propagation in the Sun, I came across a nice discussion in The Sun from Space by Kenneth R. Lang.

On pages 126 and 136, there are diagrams showing the sound paths of p modes. Moreover, the diagram on page 136 also shows woe gravity modes propagate, which exist only deep inside the sun − the outer layers do not sustain gravity modes because there are no sufficiently well-defined interfaces between regions with different properties.

And on pages 129/130, there is a discussion of the speed of sound, which comes out (from the density derivative of pressure) as roughly

c = sqrt(T/10 000 K) × 10 km/s

As the temperature rises strongly in the Sun's interior, this explains why the speed of sound increases with depth.

Zephir: As I said above, it's one of the open question. Theoretically both is possible, so experiment has to decide. As Stefan said, it's ν-less double β-decay that's the relevant signature here. Best,